as Davo posted - any retailer (ebuyer.com or amazon etc) will sell them. You can buy any 3.5" internal HDD drive that matches your other disk size (i.e. 500GB, 1TB, 2TB) and you can select from any manufacturer - the cheapest are basic desktop ones (WD Blue or Green) & go up in price to the NAS-rated versions (WD Red or Seagate Ironwolf). If you are mirroring your disks (RAID 1) then you could go cheap & buy any HDD that matches your other disk.If you have JBOD (no raid) then anything 3.5" will do.

Not sure of the rebuild procedure (if you're lucky it will be automatic) but power the unit off then take out the old drive & replace. Cheers,Lee

Thank you very much; apologize for delay in responding back. It is a single drive so no RAID, will purchase the better NAS rated drive. Really appreciate your reply, was led to believe the drive was custom. Again, thank you for your response.